President Obama, flanked by Vice President Joe Biden, makes a statement about the crisis in Syria in the Rose Garden at the White House in Washington on Saturday, Aug. 31, 2013. / Evan Vucci, AP

by Susan Page, USA TODAY

by Susan Page, USA TODAY

WASHINGTON - When he walked out of the Rose Garden on Saturday, President Obama studiously ignored the shouted question from a reporter: What happens if Congress says no?

Defying expectations that a U.S. military strike was imminent, Obama stunned nearly everyone by announcing that, while he had decided that the United States should take action to punish the Syrian regime for using chemical weapons against its own people, he would seek congressional authorization before moving forward.

In the decades since the War Powers Act was enacted, presidents have studiously avoided setting just that precedent - the expectation or requirement that they would get Congress' OK before launching military action. The danger in doing so is that he has weakened his own presidency - what happens if he doesn't want to seek congressional authorization the next time? - and even the presidency itself.

That argument is part of the reason that Ronald Reagan didn't seek congressional authorization before ordering the invasion of Grenada, why George H.W. Bush didn't seek authorization before launching military action in Panama, why Bill Clinton didn't seek authorization before ordering the bombing of Kosovo.

After the Kosovo bombing was underway, the House of Representatives split 213-213 on whether to support it. Clinton ignored them.

"While I believe I have the authority to carry out this military action without specific congressional authorization, I know that the country will be stronger if we take this course, and our actions will be even more effective," Obama said in his comments to the nation, with Vice President Biden by his side.

Obama's action is consistent with his views he expressed when he was a senator from Illinois and George W. Bush was president. And it is in line with American public opinion. In an NBC News poll released this week, nearly eight in 10 Americans said he should seek congressional authorization before striking Syria.

It also puts members of Congress and leaders in other nations on the spot to put up or shut up - to back military action or explain why not. "All of us should be accountable as we move forward, and that can only be accomplished with a vote," the president said.

But the first risk is that, having asked for a vote, Obama will have to win it.

That was a hurdle that British Prime Minister David Cameron couldn't clear this week when Parliament unexpectedly rejected his proposal to join the U.S. action. ("I understand and support Barack Obama's position on #Syria," Cameron tweeted after Obama spoke.)

The second risk is, win or lose, Obama and future presidents may have to live with the precedent that he is setting.